1. Field of the Invention
The present invention relates to a motor rotation rate detecting circuit that produces a rotation rate detection signal by differentiating a plurality of alternating signals having different phases that are output according to the rotation position of a motor. The present invention also relates to a motor driving device incorporating such a motor rotation rate detecting circuit.
2. Description of the Prior Art
Japanese Patent Applications Laid-Open Nos. H9-127140 and S57-76456 disclose conventionally known examples of motor rotation rate detecting circuits that produce a rotation rate detection signal by differentiating two-phase sinusoidal signals output from an encoder coupled to a motor.
The motor rotation rate detecting circuit disclosed in Japanese Patent Application Laid-Open No. H9-127140 includes an inverting circuit that inverts the polarities of two-phase sinusoidal signals output from an encoder coupled to a motor, a switching circuit that selects one among the four sinusoidal signals, namely the two-phase sinusoidal signals output from the encoder coupled to the motor and their respective inverted signals, a differentiating circuit that differentiates the signal output from the switching circuit, a logic selecting circuit that controls the timing of the switching performed by the switching circuit, and a differentiation error eliminating circuit that eliminates a differentiation error component from the differentiated signal output from the differentiating circuit to output a rotation rate detection signal.
Differentiating circuits of various configurations can be used in a motor rotation rate detecting circuit. However, a signal having a frequency higher than that corresponding to the maximum motor rotation rate is a high-frequency noise signal, and therefore it is customary to use a differentiating circuit that does not perform differentiating operation on a signal having a frequency higher than that corresponding to the maximum motor rotation rate.
FIG. 12 shows an example of the configuration of a differentiating circuit that does not perform differentiating operation on a signal having a frequency higher than that corresponding to the maximum motor rotation rate. The differentiating circuit shown in FIG. 12 is composed of a capacitor C3, resistors R9 and R10, and an operational amplifier OP3. One end of the capacitor C3 serves as the input end of the differentiating circuit. The other end of the capacitor C3 is connected through the resistor R9 to the inverting input terminal of the operational amplifier OP3. The non-inverting input terminal of the operational amplifier OP3 is grounded. The operational amplifier OP3 receives negative feedback through the resistor R10. The node at which the output terminal of the operational amplifier OP3 and the resistor R10 are connected together serves as the output end of the differentiating circuit. Here, let the capacitance of the capacitor C3 be C3 [F] and the resistance of the resistor R10 be R10 [Ω]. Then, the differentiating operation frequency of the differentiating circuit shown in FIG. 12 is lower than 1/ (2×π×C3×R10) [Hz]. The differentiating operation frequency denotes the frequency at which the differentiating circuit offers differentiating operation. Accordingly, the capacitance C3 and the resistance R10 are so determined that the maximum value of the differentiating operation frequency is equal to the frequency corresponding to the maximum motor rotation rate.
The motor rotation rate detecting circuit described above is used to detect the rotation rate of various types of motor. For example, it is used to detect the rotation rate of a spindle motor used in an optical disk apparatus.
In an optical disk apparatus, as the disk rotation rate becomes increasingly high, the range of the frequency of the output signal of the encoder coupled to the spindle motor becomes increasingly wide. As the range of the frequency of the output signal of the encoder becomes wider, the maximum value of the differentiating operation frequency of the differentiating circuit shown in FIG. 12 needs to be made higher.
However, raising the maximum value of the differentiating operation frequency of the differentiating circuit causes the differentiating circuit to perform differentiating operation on signals in a high-frequency range even when such signals are not needed because the motor is rotating at a low rate. As a result, when the motor is rotating at a low rate, quite inconveniently, the differentiating circuit amplifies high-frequency noise.